The present invention relates to a developing apparatus for a wet image forming apparatus which uses a developing solution in which toner is dispersed in a solvent and which is capable of removing a surplus developing solution or solvent remaining on a photosensitive material after development, divided into a non-image part and an toner part, among image forming apparatuses represented by, for example, an electrophotographic copy machine, a laser beam printer, and the like.
The wet image forming apparatus using a developing solution in which toner is dispersed uses a developer in which toner grains consisting of resin and pigment are dispersed in a mainly petroleum-based non-polar solvent and the toner is charged to a predetermined electric potential by a charge support agent added to a solvent liquid. The wet image forming apparatus attracts attention now since image quality can be more improved because of the small grain diameter of and less energy can be necessary for fixing because of the lower resin content rate of the toner, in comparison with a dry image forming apparatus (magnetic brush type development method) which has been spreading widely.
A conventional wet image forming apparatus uses an electric field transfer method as follows. In a state where a surplus developer remaining after development is squeezed slightly, i.e., where a solvent remains to some extent, the surplus developer is conveyed to a transfer area where a photosensitive drum (transfer medium) and a developing roller face each other with a predetermined gap maintained therebetween. A condition is prepared such that the gap between the roller and the transfer medium is filled with a solvent. The developer is transferred to a photosensitive member, utilizing a fact that toner causes electrophoresis due to an electric field applied to the transfer medium, under the condition described above.
In this electric field transfer method, problems lie in that the toner image can be easily disturbed during transfer, a surplus solvent moves to the transfer medium and can not be collected sufficiently, and the like.
To solve the problems, developments have been made in an offset transfer method which does not utilizes electrophoresis during transfer but utilizes heat and/or pressure to achieve transfer.
In this offset transfer method, the toner image is completely dried prior to the transfer so that the toner image cannot be disturbed. In addition, a solvent having an odor or inflammability can be collected in the first half of the image forming process. Therefore, the offset transfer method is more advantageous than the electric field transfer method.
In the offset transfer method described above, the toner image on a photosensitive member has to be dried to a desired degree during a period from development to transfer, so that a squeeze roller is used together with a drying blower in many cases.
As an example of the wet image forming apparatus using a squeeze roller, a squeeze method of squeezing away a carrier solution on the photosensitive member by using an elastic roller is disclosed in the U.S. Pat. No. 5,576,815.
The squeeze roller removes a surplus liquid film in the following manner. The squeeze roller is installed immediately behind a developing area where the photosensitive member and the developing roller face each other with a slight gap maintained from the photosensitive member, and is rotated in an against direction which is opposite to the rotation direction of the photosensitive member at the position where the squeeze roller faces the photosensitive member, at the speed in the range which is 2 to 4 times higher than the circumferential speed ratio which is a difference of the moving speed of the outer circumferential surface of the squeeze roller with respect to the moving speed of the photosensitive member.
However, to remove a much larger amount of solvent, a problem arises in that such an image defect is caused that trails the image itself backward in the rotating direction because toner grains accumulated on the latent image are removed together with the solvent and an accumulated toner layer is thereby broken.
There is a limitation to the amount of surplus developing solution (thickness of a film) that can be removed without causing the image defect. Treatments such as increasing of an output of the drying blower, heating, extension of a time up to transfer, and the like are necessary to remove a desired amount of a remaining developing solution prior to the transfer step.
However, any of the treatments will cause a problem with respect to high-speed processing or power saving.